1. Field of the Invention
The present invention relates to intruder detection systems that detect, by burying in intruder surveillance areas leaky transmission lines, whether or not intruders such as humans are present in those areas.
2. Description of the Related Art
As shown in Japanese Laid-Open Patent Publication No. 2007-179402, a conventional intruder detection system is configured as follows: A radiation-type leaky coaxial transmission line is used for leaky transmission lines each on the transmission side and the reception side, a detection signal leaked from the transmission-side leaky transmission line is received by the reception-side leaky transmission line, and an intruder is detected based on variations in the signal level of the received detection signal caused by the intruder. The radiation-type leaky coaxial transmission line is the one that uses a leaky coaxial transmission cable that propagates a radio wave in leaky wave mode, and radiates a large quantity of detection signal in a transverse direction of the transmission line (direction outward from the leaky transmission line and perpendicular to a longitudinal direction thereof).
Despite the above, because the intruder detection system observes scattering of the detection signal by an object, if a large quantity of detection signal is radiated in the transverse direction of the transmission line, scattering of the detection signal by an object moving in the transverse direction of the transmission line is enhanced. Thereby, although a detection range is required to be set to a predetermined intruder surveillance area, an actual surveillance area may sometimes become greater than the predetermined area. Therefore, there has been a problem in that, when the size of an object is extremely large, an object outside a detection area might be unnecessarily detected even if it is distant from the transmission line.
By the way, a surface-wave-type leaky coaxial transmission line is well known that generates a surface electric field in the close proximity of the transmission line (a transmission line using a leaky coaxial transmission cable that propagates a radio wave in surface wave mode); however, using of this surface-wave-type leaky coaxial transmission line has caused a problem in that the detection area becomes too small because the detection signal reaches only the close proximity of the transmission line, as well as detectable height from the transmission line thereby becomes lower compared to the predetermined intruder surveillance area.
As described above, there have been problems in that a distant unnecessary object is detected in the intruder detection system when a radiation-type leaky coaxial transmission line is used, while a detection area becomes too small in the intruder detection system when a surface-wave-type leaky coaxial transmission line is used.